Optical amplifiers provide the capability to directly amplify an optical signal without first converting the optical signal to an electrical signal. Optical amplifiers are useful as repeaters and preamplifiers in optical communication systems. A semiconductor optical amplifier is typically constructed as a modified laser diode. An optical cavity is fabricated in a substrate with facets which act as mirrors located on opposite ends of the optical cavity. When the device is appropriately biased, the optical cavity has optical gain. In order to provide operation as an amplifier rather than as a laser, antireflection coatings are applied to the facets or the facets are oriented at an angle relative to the optical axis of the cavity. An input optical signal is injected through one of the facets, is amplified in the optical cavity and passes through the facet at the opposite end of the optical cavity. Optical amplifiers are described generally by N. A. Olsson in "Lightwave Systems With Optical Amplifiers", Journal of Lightwave Technology, Vol. 7, No. 7, July 1989, pages 1017-1082.
Use of optical amplifiers for optical signal detection is disclosed by M. Ikeda in "Signal Monitoring Characteristics for Laser Diode Optical Switches", Journal of Lightwave Technology, Vol. LT-3, No. 4, August 1985, pages 909-913; D. J. Malyon et al in "Laser Amplifier Control in 280 Mbit/s Optical Transmission System", Electronics Letters, February 1989, Vol. 25, No. 3, pages 235-236; M. Gustavsson et al in "Traveling Wave Semiconductor Laser Amplifier Detectors", Journal of Lightwave Technology, Vol. 8, pages 610-617, April 1990; A. Alping et al in "100 Mbit/s Laser Diode Terminal With Optical Gain for Fiber Optic Local Area Networks", Electronics Letters, Vol. 20, No. 19, Sep. 13, 1984, pages 794-795; and A. Ellis et al in "A Novel All Electrical Scheme for Laser Amplifier Gain Control", Proceedings 14th European Conference on Optical Communications, 1988, pages 487-490. A modulated optical signal injected into the optical cavity causes a time-varying diode voltage which appears at the current input terminals. The optical amplifier thus detects the modulation of the optical signal. The detected signal can be utilized for signal monitoring, gain control, network control, and the like.
In order to increase the information carrying capability of optical communication systems, microwave multiplexing of optical signals has been proposed. In these systems, a wideband microwave signal composed of many frequency multiplexed microwave subcarriers is used to intensity modulate a high speed laser diode. The optical signal is transmitted through a conventional single mode optical fiber. The microwave subcarriers can be modulated by either analog or digital signals and can be used to carry voice, data, video, digital audio and high definition video. Bandwidths of 1 to 20 gigahertz can be achieved in subcarrier multiplexed optical communications systems.
It has been suggested in the prior art that the maximum detection bandwidth of an optical amplifier is on the order of several hundred megahertz. This would severely limit the application of optical amplifiers as detectors in subcarrier multiplexed optical communication systems, which frequently have a bandwidth in excess of one gigahertz. Accordingly, it is desirable to provide optical amplifiers having wide detection bandwidths.
It is a general object of the present invention to provide improved optical communication systems.
It is another object of the present invention to provide semiconductor optical amplifier-receiver systems having wide detection bandwidths.
It is a further object of the present invention to provide optical amplifier-receiver systems utilizing impedance transformers for providing wide detection bandwidths.
It is yet another object of the present invention to provide optical amplifier receiver systems having detection bandwidths on the order of several gigahertz.
It is a further object of the present invention to provide optical amplifier receiver systems which simultaneously function as in line optical amplifiers and wideband receivers.